Electrowinning of chromium



Patented Sept. 8, 1953 Michael C. Carosella and John D. Mettler, Niagara Falls, Y;., assi'gnors to Union Carbide and Carbon Corporation, a corporation of New York.

Application June '1', 1951, Serial No. 230,413

The present invention relates to a cyclic electrowinning process for the preparation of chromium metal fromiron-chromi'um alloys.

Heretofore, one method for the electrowinning of chromium was plating from chromic acid baths. The costly chromic acid required as feed and the high electrical power usage for this method renders it very inefiicicnt.

Another known process for the electrowinning of chromium comprises preparing ammonium chromium alum, used as the feed in the process,

by digesting chromium-ore with anolyte from the electrolysis step plus additional sulfuric acid, crystallizi-ng the iron, aluminum and other impurities from the digest liquors as alums with a resultant consumptionof sulfuric acid and am monium sulfate, and finally recovering a substantially pure chromium alum from the resultant partially purified solutions by repeated crystallizations. Although electrolytic chromium of high purity can be produced by this process, the complexity of the process as well as comparatively lowchromium recoveries'due to cocrystallization of chromium with-the discarded alums detracts from the commercial application of the process.

It is an object of the present invention to ob-' tain a cyclic process by which chromium-metalof high purity may be produced without the inefiiciencies of the processes" used heretofore.

Another object is to produce in. the cyclic process chromium chemicals which may be efficiently used in theelectrowinning step to form chromium metal of high purity.

Other advantages and aims of. the invention will be apparent from the following description. The single figure isv a fiow sheet illustrating the steps of the cyclic method of the invention. .In. accordance with the present. invention an iron-chromium alloy is digested with a "dilute mineral acid, for example sulfuric acid, at a temperature of between 60 and 105 C. This step is indicated in box I of the flow sheet. The resulting pulp is filtered to remove the residue which consists largely of silica. This step is indicated in box 2 of the flow sheet. The filtrate is added to a hot aqueous solution of an alkali or an alkaline earth metal base, for example sodium carbonate, under substantially non-oxidizing atmospheric conditions to a final pH below 3.8 to yield a granular basic chrome sulfate precipitate which is easily filtered and washed free of soluble salts. The precipitation step is indicated by box 3 and filtering step by box 4 of the flow sheet. The temperature of said alkali or Iii Claim. (Cl. 204-105) is indicated by box 8 of the flow sheet.

of this reduced anolyte is used to dissolve-further alkaline earth metal taining of a granular precipitate, since at less than C. the precipitate tends to become gelatinous. The hydrated basic chromium sulfate precipitate is dissolved with a portion of the reduced anolyte from the electrolysis step to" give trivalent basic chromium sulfate solutionswhich are used as electrolyte in the electrowinning step. Box 5 of the flow sheet indicatesthis step. "The solution so obtained is then increased incon-- centration by evaporation. This step is -indicated by box 6 of the flow sheet.

Hydrated basic chromium sulfate precipitate can be completely dissolved with considerably less than the calculated theoretical acid required to neutralize the basic portion of the precipitate to yield a solution containing basic chromium sulfate.

Electrolysis of such basic chromium sulfate solutions, with added neutral salts to increase conductivity, such as ammonium sulfateand sodium sulfate, in a diaphragm cell, where the bath temperature is maintained between 30-60-' 0., and the catholyte pH controlled between 1.5-2.4, results in the formation of chromium metal at the cathode and sulfuric acid'containing some chromic acid at the anode. In this-step free sulfuric acid is added to the catholyte to regulate" the pH. This step is indicated by'box 1of' the flow sheet. g

The hexavalent chromium in theanolyte is reduced to chromium valence-III with a common reducing agent, organic or inorganic. This step basic chromium sulfate precipitate to provide cell feed- The cyclic use of this reduced anolyte is-indicated by the line connecting box, 8 and box 5; of the flow sheet. The remaining reduced anolyte which contains the sulfate portion of the basic chromium sulfate precipitate as sulfuric acid is in excess and can be used to dissolve iron-chromium alloy from which further basic chromium sulfate can be derived. The cyclic use of this excess reduced anolyte is shown by the line between box 8 and box I of the flow sheet.

We have found in the precipitation step, under substantially non-oxidizing conditions, that above a final pH of about 3.8 the percentage of iron in the precipitate increases more rapidly for a given increase in pH. Thus, with the non oxidizing atmospheric condition of operation and the pH control by operating below the designated base should beabove so C. during this step in order" to insure the'ob A portionl pH range, the iron impurities can be effectively reduced to a minimum, thereby resulting in a precipitate that can be easily filtered.

However, it should be noted that as the final pH is reduced to obtain a decrease in iron content in the precipitate, there is a reduction in the percentage of the total chromium that is precipitated. Therefore, efliciency demands that the final pH must not be reduced to too low a value.

In the electrowinning step for a given bath temperature (in the operable range of 304 C.) there is a unique value of catholyte pH (in the range between 1.5 and 2.4), at which a maximum current eiliciency is obtained. In order to secure this maximum emciency the pH should 7 be controlled by adding free sulfuric acid to the catholyte chamber.

An example setting forth quantitatively the constituents at each stepff or one complete cycle of a typical operation under the process of this inventionis as follows 420 grams of high carbon ferrochromium (60.5% Cr, 24.2% Fe) was digested with 966 grams of make-up dilute sulfuric acid and'328 cc. of excess reduced anolyte which was composed of grams of trivalent chromium ion.

14.0 grams of ammonium sulfate, 8.0 grams ofv sodium sulfate, and 118 grams of sulfuric acid. Steam and water were added in the leaching step- The resulting pulp was filtered and 42 grams of residue removedwhich consisted largely of silica, but contained 2 grams of chromium and 1.0 gram of. iron. An analysis of the filtrate showed 296 grams of trivalent chromium ion, 100.8 grams of bivalent iron ion, 14 grams of ammonium sulfate, and 8 grams of sodium sulfate. When the filtrate was added to a hot aqueous solution containing 925 grams of 10% sodium carbonate, under the conditions herein disclosed, the 3382 grams of filtered basic chromium sulfate precipitate so formed were found to contain 2628 grams of water, 280 grams of chromium, 0.8 gram of.iron, and 116 grams of sulfate. The filtrate and washing was found to contain 16 grams of chromium, 14 grams of ammonium sulfate, 1248 grams of sodium sulfate, and 100 grams of iron.

The basic chromium sulfate was dissolved with 127 cc. of reduced anolyte which consisted of 38.3 grams of trivalent chromium ion, 55.0 grams of ammonium sulfate, 32.0 grams of sodium sulfate, and 460 grams of sulfuric acid. To this solution were added 14 grams of ammonium sulfate and 8.0 grams of sodium sulfate. The resultant 3905 cc. of solution contained 318.3 grams of trivalent chromium ion, 69 grams of ammonium sulfate, 40 grams of sodium sulfate, and 0.8 gram of iron. 2300 cc. of water were evaporated off and the resultant solution used as cell feed. After electrowinning under the conditions herein disclosed chromium metal of composition 270.3 grams chromium, 0.8 gram iron, was obtained. 1605 cc. of anolyte was produced containing 48 grams of hexavalent chromium ion, 69 grams of ammonium sulfate, 40 grams of sodium sulfate, and 714 grams of sulfuric acid. This anolyte was reduced'with 40 grams of molasses to'form455 cc. of reduced anolyte, 328 cc. of which was considered above as being used to leach the ferrochromium, and 127 cc. of which was described above as the dissolving agent for the granular basic chromium sulfate P p What is claimed is: In a cyclic process for the electrowinning of metallic chromium utilizing a compartment cell and an electrolyte prepared from iron-chromium alloys, the steps of digesting an iron-chromium alloy with dilute sulfuric acid at a temperature of between approximately 60 C. and 105 C. separating the leach; liquor from the residue, adding said leach liquor under substantially non-oxidizing atmospheric conditions to an aqueous solution containing a base selected from the group consisting of the alkalies and alkaline earth metal bases at a temperature above 60 C. until a final pH below approximately 3.8 is obtained to yield a-granular basic chromium sulfate precipitate, separating said granular basic chromium sulfate precipitate from said solution, dissolving said granular basic chromium sulfate precipitate in a solution of sulfuric acid, addin at least one neutral salt to such solution to increase its conductivity, evaporating excess water from such solution to form a cell feed, introducing said cell feed into the cathode compartment of a diaphragm electrolytic cell and electrolyzing to form metallic chromium at the oathode and to form in the anode compartment of such cell an anolyte containing sulfuric and chromic acids, treating said anolyte with a redueing agent to convert the chromium ions therein to the trivalent state, dissolving an additional portion of s'aidgranular basic chromium reduced anolytel I sulfate precipitate with a portion of said reduced anolyte, and leaching an additional portion of iron-chromium alloy with the excess of said MICHAEL (J. CAROSELLA.

JOHN D. METTLER.

References Cited in the file of this patent UNITED STATES PATENTS OTHER EFERENCES Lloyd et al., Journal of the Electrochemical Society, vol. 97 (July 1950) pp. 227-34. 

